Thermal Management: Heat Sink Materials

Heat Sink Materials Explored

In our thermal management series we took a basic look at heat sinks and their applications. These devices are used to draw heat from a source, such as a microprocessor. As it collects the heat, a thermal conductor carries the heat away onto fin type objects that quickly dissipate the heat into the air. An active heat sink will incorporate power with in it’s design such as fans to aide in cooling the processor properly, while a passive heat sink is completely mechanical and therefore it will more times than not, rely on the device to offer some type of fan to distribute air.

Heat sinks are commonly constructed of either a metallic alloy or they can be graphite foam. The alloys are typically copper or aluminum. The graphite foam is the most recent and least tested. While in early stages the foam has shown to have great thermal capacity but it has two large flaws that keep it from being the best choice. The first is that in order to attach a heat sink made of graphite foam to a microprocessor you need to solder it which can create excessive heat which does not fare well for most electrical components. Also, due to its very low tensile strength, it is frail and has a greater potential for damage when compared to copper and aluminum. To solve these problems in a basic way, the foam can be coupled or mounted on an aluminum or copper base, or carrier plate, but that limits the thermal capacity of the heat sink.

Thermal Management Performance by Material

In testing performed by Advanced Thermal Solutions,TM

[1]they took three of the possible materials and put them through a heated air test. In this test they used a thin film heater with a fan to send hot air over the sink materials at various speeds to see which had the highest thermal spreading resistance and specific thermal capacity. In the test for thermal resistance the graphite foam was shown to have had a thermal resistance of approximately 1.12 oC/W at 300 linear feet per minute (lfm); comparatively, aluminum had a resistance of 0.9 oC/W at the same speed and copper had a resistance of 0.79 oC/W. While the number differential may not seem like much, at these small values that is a 29.5% difference between Graphite Foam and Copper. In the second test they performed, copper and aluminum had a 1.637 and 1.677 specific thermal capacity at 1.5 m/s air speed; however the Graphite Foam had a specific thermal capacity of 1.746. As the speed in this test increased, all of the thermal capacities decreased, however not at the same rate. For the Graphite Foam at 3.5 m/s air speed the thermal capacity was still a respectable 0.937, while the Copper and Aluminum had specific thermal capacities of 0.710 and 0.740 respectively. Based on these results, it is easy to make the claim that the Graphite Foam is the smartest choice to use in a heat sink, but other factors still may affect the decision on which to choose.

The first factor to be considered was touched on earlier, in that the Graphite Foam is very frail and special precautions must be taken when using it for heat sinks. It can be coupled to a copper or aluminum base for support, but that will severely limit its thermal spreading resistance. Due to its requirements to ensure stability, a foam-based heat sink has yet to be used in mainstream applications. As of now, the most suitable place of use is in aerospace and military programs where performance to weight ratio is at a premium and factors such as cost or ease of use can be an afterthought.

With the downsides to graphite foam materials, that leaves the use of aluminum and copper as the most viable options for heat sinks. By utilizing a stepped base heat sink with long heat conduction paths that allow for a hotter base center, but a more desirable drop off of heat as it is distributes across the fins, creates a more efficient cooling platform. Licensed mechanical engineers using computational fluid design (CFD) are able to test the variable metals under different conditions and how they perform to justify the cost involvement in regards to the results required. In these simulations, the CFD analysis shows that copper would have a 21% increased performance over aluminum due to the reduced spreading resistance showing the benefits of using this type of design.

So as of now, the most cost efficient heat sink may be one made from aluminum, however as the demand, quality and innovation increases, copper and graphite foam materials used for heat sinks will become more prevalent within the industry.